Background:The aim of this study was to explore the effectiveness of Internet-delivered cognitive-behavioral therapy (iCBT) in treating fibromyalgia (FM) compared with an identical protocol using conventional group face-to-face CBT.Methods:Sixty participants were assigned to either (a) the waiting list group, (b) the CBT group, or (c) the iCBT group. The groups were assessed at baseline, after 10 weeks of treatment, and at 3-, 6-, and 12-month follow-ups. The primary outcome measured was the impact of FM on daily functioning, as measured by the Fibromyalgia Impact Questionnaire (FIQ). The secondary outcomes were psychological distress, depression, and cognitive variables, including self-efficacy, catastrophizing, and coping strategies.Results: In post-treatment, only the CBT group showed improvement in the primary outcome. The CBT and iCBT groups both demonstrated improvement in psychological distress, depression, catastrophizing, and utilizing relaxation as a coping strategy. The iCBT group showed an improvement in self-efficacy that was not obtained in the CBT group. CBT and iCBT were dissimilar in efficacy at follow-up. The iCBT group members improved their post-treatment scores at their 6- and 12-month follow-ups. At the 12-month follow-up, the iCBT group showed improvement over their primary outcome and catastrophizing post-treatment scores. A similar effect of CBT was expected, but the positive results observed at the post-treatment assessment were not maintained at follow-up.Conclusions: The results suggest that some factors, such as self-efficacy or catastrophizing, could be enhanced by iCBT. Specific characteristics of iCBT may potentiate the social support needed to improve treatment adherence.